Simplified Explanation of Digitizing Tablet
FIG. 1 illustrates a computer 4 and a digitizing tablet 3, which provides input to the computer. (The digitizing tablet need not be a separate element, as shown in FIG. 1, but can take the form of a transparent overlay of the display 2. This overlay configuration is frequently used in pen-based portable computers.)
The tablet-and-stylus combination provide input to the computer. A highly simplified explanation of the operation of the tablet will be given. The Inventors emphasize that this explanation is given for the purpose of illustrating general principles only, and that actual digitizing tablets are, in fact, more complex than this simple explanation would indicate. This simplified explanation is given primarily to provide a conceptual foundation for illustrating causes of errors.
FIG. 2 shows a digitizing tablet 3 and its associated stylus 6. In the tablet, there are four current-to-voltage amplifiers (labeled I-to-V), one at each corner. The stylus 6, when activated, produces a signal 9, as indicated in FIG. 3. This signal induces currents I1, I2, I3, and I4, which are detected by the I-to-V amplifiers.
The I-to-V amplifiers each produce a voltage (V) indicative of the size of its respective current (I). Processing apparatus, not shown, but known in the art, receives the voltage signals, and computes the position of the stylus 6.
The currents I1-I4 are induced because the stylus 6 acts as one plate of a capacitor. The digitizing tablet supplies the other plate: it bears a resistive surface, or grid, of a material such as indium tin oxide, which acts as the other plate.
As FIG. 4 indicates, when negative charge is applied to the tip 2 of the stylus 6, a positive charge is induced on the surface of the tablet 3. Currents I1-I4 supply this positive charge. Conversely, as in FIG. 5, a positive charge on the tip 2 of the stylus 6 induces a negative charge on the tablet. The currents I1-I4 supply this negative charge.
As FIG. 6 shows, each current can be viewed as following a direct path from the stylus to one corner of the tablet. The following reasoning allows stylus position to be inferred from the voltage signals:
1. The length of each path determines the resistance of the path. PA1 2. The resistance determines the size of the current. PA1 3. The size of the current determines the voltage produced by the amplifier. PA1 4. Each amplifier voltage indicates its respective path length. PA1 1. Position the stylus on the tablet, and compute stylus position. Assume that the stylus is positioned exactly above dot 20 in FIG. 11. The computed position will be that indicated by the X adjacent dot 20, namely, x=5.19 and y=4.09. (This computed value is given in the row associated with point 20 in Table 2, above.) PA1 2. From the computed x,y data, find the actual x,y data. FIG. 12 illustrates this procedure graphically.
A highly simplified example will illustrate.
Assume that the stylus 6 in FIG. 6 represents a voltage source applied to the tablet. The paths have the relative lengths shown in FIG. 6, and also listed in the following Table 1.
TABLE 1 ______________________________________ RELATIVE RELATIVE RELATIVE RELATIVE I-to-V PATH LENGTH RESISTANCE CURRENT VOLTAGE ______________________________________ AA 0.66 0.66 1.52 1.52 BB 1.00 1.00 1.00 1.00 CC 0.93 0.93 1.08 1.08 DD 0.55 0.55 1.82 1.82 ______________________________________
The resistance of the path depends on the length, so that the relative resistances are in proportion to the relative path lengths, as indicated in the Table's columns RELATIVE LENGTH and RELATIVE RESISTANCE.
Current equals voltage divided by resistance, V/R, so that the relative currents will be inversely proportional to the relative path lengths, as indicated in the Table's column RELATIVE CURRENT.
The I-to-V amplifiers in FIG. 2 each produce a voltage which is proportional to its respective current. Thus, the voltage outputs of the I-to-V amplifiers have the relative magnitudes indicated in the rightmost column of Table 1.
The output voltage signals can be used to compute position of the stylus.